1. Field of Invention
The present invention relates to a time adjustment device that corrects the time based on signals from a positioning information satellite such as a GPS satellite, to a timekeeping device that has the time adjustment device, and to a time adjustment method.
2. Description of Related Art
The Global Positioning System (GPS) for determining the position of a GPS receiver uses GPS satellites that circle the Earth on known orbits, and each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the GPS time) with extremely high precision.
A GPS receiver that receives signals from GPS satellites must receive the TOW (Time Of Week) signal contained in the signals from a GPS satellite in order to get the time information transmitted by the GPS satellite. The TOW signal is the GPS time, and more specifically is the number of seconds from the beginning of each week. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-H10-10251.
In order to receive signals from the GPS satellites and adjust the time based thereon, the signals from the GPS satellites must be received without passing through obstructions. If the receiver is located inside a building, however, the building itself is an obstruction and the satellite signal must be received through a window.
When the satellite signal is received through a window, the signals can be more efficiently received if the receiver selects a GPS satellite at a low angle of elevation.
Whether the elevation angle of a satellite is low or not is only known to the receiver after the almanac data containing the GPS satellite orbit information is acquired, and the receiver must therefore first acquire the almanac data.
The almanac data containing the GPS satellite orbit information is updated periodically, however, and after the almanac data is updated the receiver must again receive the GPS satellite almanac data and update the internally stored copy of the almanac data.
Furthermore, because the almanac data contains orbit information for all GPS satellites in the constellation, the receiver requires a relatively long period of time to acquire the full almanac, and power consumption by the receiver is therefore high.
High current consumption not only shortens how long the receiver can operate on a single battery charge, it also limits the batteries that the receiver can use.